1. Field of the Invention
The present invention concerns a device for coupling the light of multiple light sources, more specifically to create a uniform light bundle with two or more separate light sources.
2. Discussion of the Related Art
Such devices are known in the fields of lighting and image and movie projections, whereby a light bundle has to be directed on a reflective or transmissive light valve such as a digital micro-mirror device (DMD) or a liquid crystal (LC) panel.
Light valves and in particular reflective light valves such as digital micro-mirror devices require a relative strict control of the incoming light bundle.
Indeed, the different mirrors of a micro-mirror device can only switch over a certain angle between a position wherein light is projected on stage and a position wherein light is projected off stage, whereby said angle is usually comprised between 20° to 24°, i.e. 10° to 12° in each direction.
Said certain angle limits the angle over which the light valve may be illuminated, since an angle of incoming light that is larger than 20° or 24° leads to light shining under the micro-mirrors of the light valve, resulting in an unallowable heating of the light valve.
In case use is made of one single light source, the angle of the emitted light bundle can be corrected by means of a reflector cap behind the light source and by means of an array of lenses positioned in between the light source and the light valve.
In case multiple light sources are provided, the light of said light sources should be coupled in order to create a uniform light beam that is directed to the light valve.
It is known to couple the light of two or more light sources by means of a light pipe that typically consists of a quartz rectangular sectioned integration rod. Hereto the light bundles of the different light sources are focused on an incident surface at one end of the rod. Since the light bundles are than kept in the rod by total internal reflection on the longitudinal side walls of said rod, a uniform light bundle is created leaving the integration rod at its transversal wall opposite to the incident surface.
In order to focus the light bundle of the different light sources on the incident surface of the integration rod different options are known.
A first option is to provide two light sources, two reflector surfaces and a condenser lens. The light emitted by a first of said light sources reflected by a first of said reflector surfaces towards the condenser lens. Likewise, the light emitted by the second light source is reflected by said second reflector lens towards the condenser lens. The condenser lens finally focuses the incoming light on the incident surface of the integration rod.
An inconvenience of this option is that it appears to be very difficult, if not impossible, to perfectly focus the light of the light sources on the incident surface of the integration rod. A consequence of an imperfect focus is that the light bundle leaving the integration rod has a relatively broad angle that should be narrowed again before illuminating the light valve in order to avoid excessive heat production.
The inconvenience of the above mentioned embodiment can be solved by providing an elliptic housing, so that the emitted light bundle is convergent and that, as such, the emitted light can be focused on the incident surface without the need for a condenser lens.
This embodiment has the inconvenience that the incident surface of the integration rod may not be much larger than the spot at the focal point of one of said light sources, since otherwise the efficiency of said rod is rather low.
In order to further increase the efficiency, one might use a conical integration rod, though a decrease of the area of the incident surface of the rod reduces the light coupling capacity of said rod.
Another option is to provide two light sources that are directed directly to the rod.
An inconvenience of this option is that the angles of the different light bundles entering the rod all differ, resulting in a light bundle with a wide angle leaving the rod.
Yet another option consists of providing two additional integration rods positioned one next to the other, with one end against the incident surface of the integration rod. The light sources are in this case focused on a longitudinal incident surface at the free end of said additional rods.
The wall opposite to said incident surface of both additional rods is bevelled and provided with an internal mirror, such that light entering the additional rods is reflected under a desired angle by said mirror.
The light bundles of both separate additional rods will finally be coupled in the integration rod, so that a uniform light bundle leaves the integration rod with an appropriate angle.
A major inconvenience of this last embodiment is that the mirrors tend to heat due to the incoming light. The cooling of this mirror that is positioned next to the rod however is nearly impossible due to the poor heat transport in the rod.
Another important inconvenience is that the additional rods have to be physically separated by a layer of air, since any contact between both additional rods disturbs the internal reflection in said rods.
Said physical separation of both additional rods is very difficult to realise since the dimensions of the cross section of an integration rod to be used with a conventional DMD are about 4×6 mm. This implies that the cross sectional dimensions of the additional rods have to be smaller than 2×3 mm, while the length of the additional rods may be several centimetres.